快猫短视频

IT TOOK the industrial steel makers of Sheffield a thousand years to catch up
with the Saxons of Southampton, says archaeologist Gerry McDonnell of the
University of Bradford. Saxon blacksmiths in the Dark Ages developed the same
sort of high-quality steel that made Sheffield famous during the Industrial
Revolution. The finding 鈥渢urns the conventional idea about early iron-making on
its head鈥�, he says.

In the 1740s, Benjamin Huntsman found that he could purify steel by melting
it and allowing the slag to rise to the surface so it could be skimmed off. The
resulting high-quality steel, an alloy of iron with about 2 per cent carbon, was
perfect for making watch springs, which was Huntsman鈥檚 profession. But the
discovery of small steel ingots and steel-edged knives in Hamwic, a Saxon port
buried under Southampton, proves that blacksmiths made 鈥淪heffield鈥� steel in the
middle of the Dark Ages.

McDonnell believes that the Saxon smiths used a two-stage process to make the
steel. First they dropped iron ore and charcoal into a small 鈥渂loomery鈥�, a clay
furnace about 1.5 metres high. Heating this mixture produced cast iron鈥攁
compound with about 4 per cent carbon.

Secondly, the smiths heated the cast iron on a hearth, using bellows to pump
air over the iron. The iron melts at 1100 掳C but as the temperature
continues to climb, the air from the bellows oxidises the carbon, which escapes
as carbon dioxide. When the molten metal has reached 1200 掳C, and its carbon
content has fallen to 2 per cent, it suddenly solidifies. This is because the
decrease in carbon raises the metal鈥檚 melting temperature. The result was a blob
of steel 鈥渁bout the size of a currant bun鈥�, says McDonnell, which could then be
worked into knives or other tools.

The ancient world did have other ways of making steel but none of these
produced a homogeneous lump of steel of this quality, says McDonnell. Analyses
carried out at Bradford show that the steel from Hamwic is two to three times as
hard as steel made by other techniques of the time.

Some archaeologists doubt the Saxons had the technology to reach temperatures
high enough to melt iron and produce high-quality steel. The blast furnace was
not introduced to Europe until the 15th century. When solidified lumps of molten
iron were found at other ancient sites, researchers dismissed them as mistakes
or more recent contamination.

Hamwic was only occupied in the 8th and 9th centuries, so later contamination
can be ruled out. Paul Craddock, a metallurgist at the British Museum in London,
says: 鈥淚t鈥檚 not impossible. There are big advances being made in our
understanding of iron and steel in the Roman and early medieval period.鈥�

So why was the secret of modern steel lost for a thousand years? The most
likely answer, says McDonnell, is that it was only made in small quantities and
was very expensive. When the demand for steel increased in the Middle Ages, mass
production of poor-quality metal forced out the higher-quality product.